Automatic transmissions that control the speed change operations of a vehicle normally perform gear shifts using a hydraulic pressure. This hydraulic pressure is generated by a hydraulic fluid discharged from an oil pump driven by the engine. A flow rate control valve is provided at a discharge port of the oil pump so as to supply the hydraulic fluid at a flow rate lying within fixed limits to the automatic transmission. This type of flow rate valve may also be applied for providing control pressure to a continuously variable transmission, which is, for example, disclosed in U.S. Pat. No. 5,435,795.
Such a flow rate control valve has a structure that drains excess hydraulic fluid according to the displacement of a cylindrical spool in response to the pressure of the hydraulic fluid. The spool is housed in a casing, and is supported by a spring in a direction opposite to the direction in which the pressure of the hydraulic fluid acts.
The fluid discharged by the pump is led into a space formed in the spool, and then from an orifice formed at the end of the spool. The fluid flows to the automatic transmission via an outflow port formed in the casing.
Drain holes are pierced on the periphery of the spool, which are connected to the interior space of the spool, and drain ports are formed relative to these drain holes in the casing according to a slide position of the spool.
When the discharge pressure of the pump is low, the drain holes and the drain ports do not overlap, so effectively all of the hydraulic fluid is supplied from the orifice to the automatic transmission. As the discharge pressure increases and the spool slides inside the casing, the drain holes and drain ports start to overlap, so part of the hydraulic fluid flows out from the drain ports to the intake side of the oil pump. This outflow amount is larger the higher the discharge pressure, i.e., the higher the discharge flow rate.
The flow rate of hydraulic fluid supplied to the automatic transmission from the orifice is therefore maintained almost constant according to the increase of the drain flow rate as the pump discharge flow rate increases.
However, when the discharge flow rate of the oil pump rises to a high value, the spool displaces beyond the point at which the overlap between the drain holes and drain port is maximized. Beyond this point, the drain flow rate conversely starts to decrease with increase of spool displacement distance. This decrease of drain flow rate causes an increase of orifice flow rate, and the hydraulic fluid flow rate supplied by the flow control valve to the automatic transmission exceeds tolerance limits.
The equilibrium position of the spool is a position in which the spring load is balanced against the pressure difference across the orifice. However, when the effective cross-sectional area of the drain changes over at the point from increasing to decreasing relative to the spool position as described above, there will be two spool positions in which forces are balanced for the same pump discharge flow rate. As a result, even at the same pump discharge pressure, the spool balance position when the discharge pressure is increasing and the spool balance position when the discharge pressure is decreasing will not necessarily be the same, and this may lead to hysteresis of flow rate characteristics of the flow rate control valve.